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Astron. Astrophys. 339, L41-L44 (1998)

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3. Discussion

The period and spectral properties of 1SAX J1324.4-6200 are common to both accreting magnetized neutron star X-ray pulsars (XRP) and accreting magnetized white dwarfs in intermediate polar (IP) systems. The limit on [FORMULA] s s-1 from these observations is insufficient to distinguish between neutron star and white dwarf models. If 1SAX J1324.4-6200 is an IP system in, or behind, the dark cloud DC 306.8+0.6, its luminosity is above the average of the short spin period (33-206 s) systems (Patterson 1994). This discrepancy could be resolved, if the measured absorption is local (other IP systems show high N[FORMULA]) and the system closer. Patterson (1994) shows that if spin equilibrium is assumed for IP systems, then the magnetic moment and spin period are correlated. Assuming that the measured period is at equilibrium for a magnetic moment of [FORMULA] G cm-3, the luminosity derived using the Patterson (1994) relation is [FORMULA] erg s-1. This is above the range of [FORMULA] erg s-1 expected for IP systems. Some IP such as GK Per (Watson et al. 1985) show outbursts with luminosities similar to 1SAX J1324.4-6200. However, it seems unlikely that the source was in outburst for the three years covered by the ASCA and BeppoSAX observations.

If 1SAX J1324.4-6200 is behind the dark cloud the measured luminosity is instead consistent with the XRP luminosity distribution ([FORMULA] erg s-1). An equilibrium period of 170 s implies a magnetic field of [FORMULA][FORMULA] G (Bildsten et al. 1997), which is within the expected range for XRP. The population of accretion-powered XRP has greatly increased, due to observations with the more sensitive detectors on ROSAT, ASCA, RXTE and BeppoSAX. Bildsten et al. (1997) list 44 X-ray pulsars and at least 6 others have been recently discovered (Israel et al. 1998; Kinugasa et al 1998; Corbet et al 1998; Marshall et al. 1998; Wijnands & van der Klis 1998; Hulleman et al. 1998). Most XRP are high-mass X-ray binaries (HMXRB) with five exceptions (4U 1626-67, Her X-1, GX1+4, GRO J1744-28, and SAX J1808.4-3658), which are low-mass systems. 1SAX J1324.4-6200 is unlikely to be one of the exceptions since they are more luminous and show different types of variability such as flares, bursting and transient behavior.

Roche-lobe filling (Cen X-3) and fed-wind supergiant (Vela X-1) HMXRB are the most luminous ([FORMULA][FORMULA] erg s-1) XRPs. They shows marked X-ray intensity variability, due either to eclipses or to inhomogeneities in the companion's wind. Unless both X-ray observations took place during unusually low states of 1SAX J1324.4-6200, a supergiant companion seems unlikely because of the low luminosity and lack of variability. More than half of the HMXRB are associated with Be star companions and typically show transient behavior. This profusion of Be/X-ray pulsar systems is unsurprising. Van Paradjis & McClintock (1995) predict a ratio [FORMULA]100 of Be/X-ray binaries to HMXRB with evolved companions. Therefore many more Be/X-ray binaries are likely to be discovered in galactic plane surveys by ASCA and BeppoSAX. The luminosity of Be-star systems during outbursts can change dramatically from [FORMULA] to [FORMULA] erg s-1, whereas persistent Be system such as X Per, or Be systems in quiescence, have more modest luminosities of [FORMULA][FORMULA] erg s-1, similar to 1SAX J1324.4-6200. X Per (e.g. Schlegel et al. 1993) and 1SAX J1324.4-6200 have similar overall spectral shapes and neither show strong energy-dependent pulse profiles nor evidence for Fe K line emission. Be/X-ray systems display a correlation between their spin and orbital periods (Corbet 1986; Bildsten et al. 1997) which implies an orbital period of [FORMULA]100 days for 1SAX J1324.4-6200. While 1SAX J1324.4-6200 is more likely to host a neutron star than a white dwarf, a change in the spin period of [FORMULA] yr-1 (similar to X Per, Haberl 1994), expected because of the lower moment of inertia of a neutron star, is needed to confirm this.

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© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998